Non contrast-enhanced magnetic resonance angiography (NE-MRA) is useful in the evaluation of vascular pathologies, especially in patients with impaired renal function. Most of the NE-MRA techniques rely on blood flow into a sequential series of 20 slices or 30 blocks until the entire vascular territory of interest is imaged. This process of sequential acquisition can potentially result in long scan times for the patient.
Many techniques exist to accelerate image acquisition to thereby reduce patient examination time. These exploit different aspects inherent in MR data. For instance, the use of phased-array coils provides additional information encoded in spatially varying sensitivity of the individual coil elements; this can be used to extract more information from the same data. More advanced techniques have been developed that simultaneously excite multiple spatially-separated slices, and then use coil sensitivities to extract the information in each individual slice; these are collectively called simultaneous multi-slice (SMS) imaging.
In addition to maximizing the signal from blood vessels, NE-MRA requires the minimization of signal from other tissues (e.g., background muscles, fat, etc.) so as to improve conspicuity of vessels. Often, radiofrequency suppression pulses are used for this purpose; these pulses suppress the magnetization from the background tissues, thus minimizing the signal emanating from them.
As mentioned above, conventional NE-MRA techniques rely on inflow of blood to generate the required contrast—accentuation of arteries and suppression of veins and background. One such NE-MRA technique is called quiescent interval slice-selective (QISS). In this approach, three different preparation pulses are used to suppress background tissue, venous signal, and fat signal. Quiescent interval (QI) is a time interval during which no activity takes place so as to permit inflow of sufficient unsuppressed blood into the slice of interest. The MR signal acquired after the QI time represents primarily arterial signal. The process is then repeated for all slices, until the entire vascular anatomy of interest is covered. The series of slices are then stacked together to depict the vascular tree.
The use of SMS is attractive for accelerating data acquisition of NE-MRA techniques such as QISS; however, it requires redesign of spatial and temporal application of magnetization preparation pulses to accomplish similar images in less time. Thus, it is desired to provide techniques for combining NE-MRA with SMS and magnetization preparation pulses in a manner that minimizes the overall acquisition time.